For reasons of state-of-charge (SOC) management, system efficiency, and state of health management (SOH), it may be advantageous to adjust the current in an individual battery to be higher, lower or the same as other parallel-connected batteries.
One method of controlling current between parallel batteries is by controlling a switching device connected between each of the batteries and the other parallel connected batteries. Current control involving the hard switching of batteries has quite a few disadvantages. Primarily, the current into and out of the battery cannot be finely controlled. The current is either ON or OFF, not anywhere in between. For example, if the system detects current into one of the parallel batteries as being too high, the only mechanism for control is to open a series switch to turn OFF the current. Another problem with this method is that whenever a parallel battery is disconnected from the others, its voltage will be different from that of the others. When a reconnection is made, the difference in voltage will force current in or out of the connecting batteries. If the difference is large enough, excessive and possibly damaging current inrushes can occur, thus damaging the battery, switching devices or other interconnecting hardware.
Other methods of controlling the current coming in and out of a battery involve a more active means such as controlling a regulator which actively controls the power flow in and out of each battery. The use of a regulator to control the current in and out of each of the parallel batteries addresses the discontinuity issues noted above; however, its disadvantages are high cost, weight, size and system complexity and lower reliability. The regulator often does employ switching devices, which switch at a relatively high frequency and controlled duty cycle and use magnetic or capacitive components to smooth out the effects of the hard switching. The net result is that the current into or out of each battery is smoothly controlled between the ranges of completely ON and completely OFF. The devices required to perform the switching, smoothing and controlling actions in these regulators have cost, size and complexity that is proportional to their power handling capability. This means that the regulators are often a significant portion of the physical and cost budget of the overall energy storage system. In addition, the added complexity and increased parts count represents a liability in reliability over systems without such hardware.
Still another method employs a series resistor between the batteries and the connecting bus, with a value high enough so that it controls the current into the batteries and controls the ratio of current into each. The use of simple resistive elements to control the current into parallel batteries is a simpler mechanism and lower cost and size than using a regulator for continuous current control, but its disadvantage is that the resistors waste a lot of useful energy while controlling the current going into each of the batteries. The energy wasted in each of the resistors is proportional to the square of the current going through them. This energy is permanently lost in the form of heat which must in turn be dissipated safely within the system to the environment. This lost energy reduces the overall storage system efficiency and the heat can reduce the overall system reliability if it contributes to the warming of the batteries and other devices. In addition, the loss of energy reduces the charge time for each of the batteries for a given size of charging system.
Another approach involves controlling the current using magnetic current controlling devices called Saturable Reactors. These devices can be externally controlled to limit the current through each of the parallel strings. The use of magnetic components to control the power into and out of the parallel batteries solves the problem of lost energy into each of the batteries and extended charge time during normal conditions, i.e. balanced operation. In addition, the control mechanisms can potentially be simpler than that of a regulator and still control the power better than a simple on-off switch. However, the magnetic devices must be sized large enough to accommodate worse case current handling. This adds to the cost and size of the overall system.